1. Field of the Invention
The present invention relates to image reproduction machines, such as printers and copiers, and more particularly relates to paper feed apparatus for such machines.
2. Description of Related Art
In conventional image reproduction machines such as, for example, laser printers, cut paper sheets to be imprinted with the reproduced image are fed through printing means within a housing portion of the machine, and then discharged from the housing into an external paper receiving well formed in the housing. In a laser printer, these printing means include a rotating photoconductive drum from which toner, in the pattern of the image to be reproduced, is electrically transferred onto the moving sheet. As is moves away from the drum, the sheet is passed through a fuser structure which, by a combination of heat and mechanical pressure, fuses the applied toner to the sheet.
Upon exiting the fuser structure the sheet is guided through a curved path, typically around a guide roller, to a spaced plurality of exit roller sets that frictionally drive the imprinted sheet through and then horizontally discharge it, in a forward direction, from a horizontally elongated housing outlet opening into the paper receiving well. The bottom side of the well typically has a horizontal paper support surface spaced forwardly apart and downwardly offset from the outlet opening. Extending rearwardly from the back edge of this horizontal paper support surface is a rearwardly and downwardly ramped surface forming a rearward extension of the horizontal support surface.
The exit roller sets each comprise a rotationally driven resilient roller that pinches the sheet against an idler roller whose axis is upwardly and rearwardly offset from the axis of the driven roller. In theory, the exit roller structure of the printer is positioned relative to the overall bottom side of the receiving well in a manner such that a leading edge portion of the first discharged sheet clears the depressed rear area of the well before its natural downward bend causes it to contact and slide forwardly along the horizontal support surface as the balance of the sheet is discharged from the housing outlet opening.
When the first sheet is fully discharged, a rear end portion thereof bends downwardly into the depressed rear well area and comes to rest on the ramped bottom side surface thereof. Each subsequently discharged sheet follows this discharge sequence, but contacts the previously discharged sheet instead of contacting the bottom side surface of the well, so that a bent stack of discharged sheets is progressively formed in the well area of the housing.
While this is the intended discharge path of each imprinted sheet, a potential paper curling problem can distort the discharge path of the sheets such that they simply roll up in the depressed rear well area, thereby preventing the intended bent stacking of the discharged sheets. This paper curling problem is particularly pronounced in instances where relatively light weight paper is being used, and where there is a sharp guide path bend at the exit of the fuser structure, and can cause the leading edge of the first exiting sheet to bend downwardly to an extent that it strikes the ramped well surface instead of the horizontal well surface in front of it.
When this occurs, the sheet simply bends into a rolled configuration and undesirably remains in the depressed well area. The leading edges of successively discharged sheets similarly strike the ramped well surface, or previously rolled sheets as the case may be, and quickly build up to block the paper discharge path.
One previously proposed solution to this problem has been to radially enlarge one relatively thin end portion of each of the driven exit rollers so that as each sheet is pinched between and driven forwardly by the exit roller sets these radially enlarged roller end portions form on the underside of the driven sheet relatively small corrugation lines along the entire length of the sheet. This permanent corrugation of the sheet tends to stiffen it sufficiently so that as it is discharged from the printer housing its leading edge clears the ramped well surface and properly lands on the horizontal well surface in front of it, thereby forming the desired bent stack of discharged sheets in the well area instead of forming a disorderly array of rolled sheets in the depressed rear well portion.
While this permanent sheet corrugation method tends to solve the aforementioned paper curling problem, it often creates a new problem--namely, the "crinkling" of the sheets as they are discharged from the printer housing outlet opening. Specifically, by positioning the thin, disc-shaped corrugating structures immediately adjacent the paper "pinch" zones of the exit roller sets, the sheets are subjected to relatively large side-to-side shortening forces at the points at which they traverse the "nip" areas of the exit roller sets. Accordingly, longitudinal portions of the sheets are forcibly caused to slide longitudinally within the nip areas of the roller sets, thereby crinkling the sheets.
Another limitation commonly associated with conventionally configured image reproduction machines, such as the laser printer discussed above, relates to the maximum number of discharged sheets that may be stacked in the aforementioned bent configuration in the housing well area before the stack blocks the external paper discharge path and must be removed from the well. It will be appreciated from the general well geometry described above that the rear edge of the horizontal bottom well surface must be close enough to the housing outlet opening to assure that the leading edges of the discharging sheets forwardly clear such rear edge before they bend down to a level below that of the horizontal well surface. Otherwise, the previously described sheet roll-up problem will occur.
Of course, the closer this rear edge is positioned to the housing outlet opening, the less likely it is that such roll-up will occur. However, as this rear edge is moved closer to the outlet opening a corresponding decrease in the minimum distance between the ramped well surface and the outlet opening also occurs. While the available stack height directly above the horizontal surface of the open-topped well is not theoretically limited, the maximum number of discharged sheets that may be stacked in the well is limited by this distance between the ramped well surface and the outlet opening. Specifically, as the discharged paper stack grows, at some point the bent rear portion of the top sheet interferes with the discharge of the next sheet, and the stack must be removed from the well before subsequent sheets can be discharged thereto.
In printers, and other types of image reproduction machines having this conventional well and paper discharge design, the optimal front-to-rear placement of the rear edge of the horizontal well surface tends to result in an outlet opening-to-ramped well surface dimension which limits the maximum number of discharged sheets that can be received in the well area to a number less than 500 (i.e., the number of cut paper sheets in a standard one ream package). As an example, a conventional printer of the general type described above typically has a discharge stack capacity of from about 425 to about 450 sheets--i.e., a number substantially short of a more desirable 500 sheet stack capacity.
Another problem that conventional printers and other types of image reproduction machines of this well configuration tend to have is related to their pivotally mounted paper output sensor member that is positioned outwardly adjacent the housing outlet opening and functions to monitor the number of sheets in a given discharge stack thereof. The sensor, typically a small plastic molding, is pivoted upwardly by each discharged sheet and then pivots downwardly to rest upon the top side of the stack until this pivot cycle is initiated again by the next discharged sheet. Particularly when the discharged paper stack is relatively thick, the sensor is subject to being forced upwardly and broken by the stack as the stack is removed from the well by lifting it upwardly and rearwardly therefrom.
It can readily be seen from the foregoing that it would be desirable to provide an image reproduction machine of the general type described with paper discharge apparatus that eliminates or at least substantially reduces the above mentioned discharge problems, limitations and disadvantages. It is accordingly an object of the present invention to provide such apparatus.